Rocket assisted projectiles are artillery or cannon fired projectiles that have an integrated rocket motor for extending the effective range of the projectile. Conventional artillery or cannon projectiles rely solely on the gases generated from the ignited propellant charge to propel the projectile to the intended target. With rocket assisted projectiles, a propellant charge is used to eject the projectile from the barrel of a mortar or cannon before the rocket motor is ignited to propel the projectile to the target. The rocket motor can serve as the primary means of conveying the projectile to the target or supplement the propellant charge. Rocket assisted projectiles can often also include adjustable vanes and Guidance, Navigation and Control (GNC) systems that can be operated in conjunction with the rocket motor to guide the projectile to the target increasing the accuracy of the projectiles.
Rocket assisted projectiles are initially launched by a propellant charge that propels the projectile at least clear of the barrel muzzle before the rocket motor is ignited. The timing of the ignition of the rocket motor is critical to the safe and effective operation of the projectile. If the rocket motor ignites too early, while the projectile is still travelling down the barrel, the combined propellant gases from the ignited propellant charge and rocket motor can overpressure the barrel possibly deforming and even rupturing the barrel. Similarly, if the rocket motor ignites too late, the projectile may no longer be properly oriented or too late in its flight for the rocket motor to be effective. In addition, the propellant charge for rocket assisted projectiles is often smaller than conventional propellant charges for similarly sized projectiles as the propellant charge is often only used to eject the projectile from the barrel and the primary source of propellant is the rocket motor. If the rocket motor ignites too late or fails to ignite, the projectile may hit the ground near the artillery or cannon prior to the rocket motor being ignited and potentially detonate or otherwise cause undesired harm.
Accordingly, the ignition system for the rocket motor must consistently ignite the rocket motor at the proper time following the firing of the projectile. In addition, the ignition system must be shelf stable as the projectile may be stockpiled for years and possibly even decades before the projectile is actually fired. A common timing mechanism is an elongated pyrotechnic chain linking the propellant charge with the rocket motor. The pyrotechnic chain is ignited at one end when the propellant charge is ignited. The pyrotechnic chain burns along the chain and ignites the rocket motor when the pyrotechnic chain is burned through. Overtime, the chemicals in the chain can break down or impurities can be introduced into the chain affecting the rate at which the chain burns and correspondingly altering the timing of the ignition of the rocket motor. In addition, the chain may be inadvertently ignited at a point along the chain rather than at the end causing the rocket motor to be ignited earlier than if the chain were properly ignited at the end.
A similar timing mechanism is a battery powered electrical circuit with an electronic timer that ignites the rocket motor at the predetermined time. However, if the timing mechanism malfunctions, the circuit can prematurely discharge, igniting the rocket motor. Similarly, the batteries can lose charge over time such that the igniter discharge is insufficient to ignite rocket motor. As such, battery powered timing and igniters require substantial and regular maintenance to ensure the ignition system is operating properly after extended storage.
As such, there is a need for shelf stable means of igniting the rocket motor of rocket assisted projectiles that will not degrade over time or require substantial maintenance.